A variety of methods have been utilized to obtain compounds in stereochemically pure form. While certain diastereomers and enantiomers can be synthesized using asymmetric synthetic techniques, not all compounds can be obtained in this manner. Moreover, such syntheses often require expensive reagents. Altemnatively, diastereomers can be obtained by selective recrystallization of one diastereomer. In some instances, selective recrystallization can also be used to prepare an enantiomer. The enantiomer must first be converted to a diastereomer by reacting it with a chiral auxiliary, then one diastereomer can be selectively recrystallized. After recrystallization the chiral auxiliary is removed to give one enantiomer. Selective recrystallization, however, is not suitable for the preparation of all compounds. In addition, it is considered inefficient, in that product recovery is often low and purity uncertain.
Diastereomers can also be resolved chromatographically, although the large amount of solvent required for conventional preparative chromatography results in the preparation of relatively dilute products. Moreover, limited throughput makes conventional methods impractical for large-scale production. Enantiomers can also be separated chromatographically when a chiral solid support is used.
A very complex chromatography process, simulated moving bed chromatography (SMB), has been applied to the large-scale separation of C8 hydrocarbons (Broughton, D. B., Chem. Eng. Prog. (1968), 68:6).; the separation of fructose and glucose by adsorption on a zeolite solid phase (Kieprathipanja, S., U.S. Pat. No. 5,000,794); and also to the separation of enantiomers using a chiral solid support (Gattuso, M. J., et al., Chemistry Today (1996), 17 and Gattuso, M. J., U.S. Pat. No. 5,889,186 (1999)). However, the effective application of simulated moving bed technology to the separation of any specific group of chemical compounds is quite unpredictable. This is particularly true when the compounds to be separated are closely related structurally and are intended for pharmaceutical use, as are the stereoisomers of 2xe2x80x2,3xe2x80x2-dideoxy-5-fluoro-3xe2x80x2thiocytadines derivatives (hereinafter xe2x80x9cFTCxe2x80x9d). FTC derivatives, particularly the L or (xe2x88x92) enantiomer of cis-FTC alcohol, have been shown to exhibit therapeutic antiviral effects.
Thus, an effective method of preparing stereochemically pure compounds which are FTC derivatives would be very useful.
The present invention relates to a method of preparing a cis or a trans diastereomer of a 2xe2x80x2,3xe2x80x2-dideoxy-5-fluoro-3xe2x80x2thiocytadine derivative represented by Structural Formula I: 
In Structural Formula I, R is H, a substituted or unsubstituted organic acid radical, a substituted or unsubstituted alkyl group, a substituted or unsubstituted aryl group, a substituted or unsubstituted heteroaryl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted heteroaralkyl group, a substituted or unsubstituted cycloalkyl group, a substituted or unsubstituted heterocycloalkyl group, a substituted or unsubstituted cycloalkylalkyl, a substituted or unsubstituted heterocycloalkylalkyl, a sugar or a protecting group. The method involves forming a solution of the cis and the trans diastereomers of the 2xe2x80x2,3xe2x80x2-dideoxy-5-fluoro-3xe2x80x2thiocytadine derivative, then separating the cis and the trans diastereomers of the 2xe2x80x2,3xe2x80x2-dideoxy-5-fluoro-3xe2x80x2thiocytadine derivative by simulated moving bed chromatography to obtain at least one diastereomer. In one embodiment, the cis and the trans diastereomers of the 2xe2x80x2,3xe2x80x2-dideoxy-5-fluoro-3xe2x80x2thiocytadine derivative are each recovered in at least 95% diastereomeric excess. In another embodiment, the cis diastereomer of the 2xe2x80x2,3xe2x80x2-dideoxy-5-fluoro-3xe2x80x2thiocytadine derivative is recovered in at least 95% diastereomeric excess, and the trans diastereomer of the 2xe2x80x2,3xe2x80x2-dideoxy-5-fluoro-3xe2x80x2thiocytadine derivative or a mixture containing the trans diastereomer of the 2xe2x80x2,3xe2x80x2-dideoxy-5-fluoro-3xe2x80x2thiocytadine derivative is recovered.
In another embodiment, the invention relates to a method of preparing an enantiomer of a 5-fluoro-1-{2-(hydroxymethyl)-1,3-oxathiolan-5-yl}cytosine. The method involves reacting racemic 5-fluoro-1-{2-(hydroxymethyl)-1,3-oxathiolan-5-yl}cytosine with a chiral auxiliary to form a mixture of diastereomers represented by Structural Formula II: 
In Structural Formula II, R1 is a chiral auxiliary. The mixture of diastereomers formed by reacting 5-fluoro-1-{2-(hydroxymethyl)-1,3-oxathiolan-5-yl}cytosine with a chiral auxiliary is separated by simulated moving bed chromatography to obtain at least one diastereomer. The chiral auxiliary is then removed from at least one diastereomer obtained by the simulated moving bed separation to form an enantiomer of a 5-fluoro-1-{2-(hydroxymethyl)-1,3-oxathiolan-5-yl}cytosine. In one embodiment, at least one of the diastereomers of the 2xe2x80x2,3xe2x80x2-dideoxy-5-fluoro-3xe2x80x2thiocytadine derivative is recovered in at least 95% diastereomeric excess.
In another embodiment, the invention relates to a method of preparing an enantiomer of a cis-2xe2x80x2,3xe2x80x2-dideoxy-5-fluoro-3xe2x80x2thiocytadine or a trans-2xe2x80x2,3xe2x80x2-dideoxy-5-fluoro-3xe2x80x2thiocytadine derivative represented by Structural Formula I. The method involves forming a solution containing a first and a second enantiomer of the cis-2xe2x80x2,3xe2x80x2-dideoxy-5-fluoro-3xe2x80x2thiocytadine derivative or a solution containing a first and a second enantiomer of the trans-2xe2x80x2,3xe2x80x2-dideoxy-5-fluoro-3xe2x80x2thiocytadine derivative. The first and second enantiomer of the cis-2xe2x80x2,3xe2x80x2-dideoxy-5-fluoro-3xe2x80x2thiocytadine derivative or the first and second enantiomer of the trans-2xe2x80x2,3xe2x80x2-dideoxy-5-fluoro-3xe2x80x2thiocytadine derivative are then separated by simulated moving bcd chromatography using a chiral solid support to obtain at least one of the enantiomers of the cis-2xe2x80x2,3xe2x80x2-dideoxy-5-fluoro-3xe2x80x2thiocytadine derivative or at least one of the cnantiomers of the trans-2xe2x80x2,3xe2x80x2-dideoxy-5-fluoro-3xe2x80x2thiocytadine derivative.
In another embodiment, the invention relates to a method of preparing an enantiomer of cis-{5-(4-amino-5-fluoro-2-oxo-1(2H)-pyrimidinyl)-1,3-oxathiolan-2-yl}methyl butanoate represented by Structural Formula III: 
The method involves forming a solution containing a first and a second enantiomer of cis-{5-(4-amino-5-fluoro-2-oxo-1(2H)-pyrimidinyl)-1,3-oxathiolan-2-yl}methy butanoate. The first and second enantiomer of cis-{5-(4-amino-5-fluoro-2-oxo-1(2H)-pyrimidinyl)-1,3-oxathiolan-2-yl}methyl butanoate are then separated by simulated moving bed chromatography using a chiral solid support to obtain at least one of the enantiomers of cis-{5-(4-amino-5-fluoro-2-oxo-1(2H)-pyrimidinyl)-1,3-oxathiolan-2-yl}methyl butanoate.
In another embodiment, the invention involves a method of preparing a cis-5-fluoro-1-{2-(hydroxymethyl)-1,3-oxathiolan-5-yl}cytosine enantiomer represented by Structural Formula IV: 
The method involves forming a solution of a fust and a second enantiomer of cis-5-fluoro-1-{2-(hydroxymethyl)-1,3-oxathiolan-5-yl}cytosine. The first and second enantiomer of cis-5-fluoro-1-{2-(hydroxymethyl)-1,3-oxathiolan-5-yl}cytosine are then separated by simulated moving bed chromatography using a chiral solid support to obtain at least one of the enantiomers of cis-5-fluoro-1-{2-(hydroxymethyl)-1,3-oxathiolan-5-yl}cytosine. Preferably, the enantiomer which is obtained is (xe2x88x92) cis-5-fluoro-1-{2-(hydroxymethyl)-1,3-oxathiolan-5-yl}cytosine.
When the method of the invention involves separating enantiomers of a 2xe2x80x2,3xe2x80x2-dideoxy-5-fluoro-3xe2x80x2thiocytadine derivative by simulated moving bed chromatography, the method can firther include a step of contacting the second enantiomer of the 2xe2x80x2,3xe2x80x2-dideoxy-5-fluoro-3xe2x80x2thiocytadine derivative or a mixture containing the second enantiomer of the 2xe2x80x2,3xe2x80x2-dideoxy-5-fluoro-3xe2x80x2thiocytadine derivative with a base to reform a mixture containing the first enantiomer of the 2xe2x80x2,3xe2x80x2-dideoxy-5-fluoro-3xe2x80x2thiocytadine derivative and the second enantiomer of the 2xe2x80x2,3xe2x80x2-dideoxy-5-fluoro-3xe2x80x2thiocytadine derivative. Suitable bases include sodium hydride, an alkyl lithium such as n-butyl lithium, potassium t-butoxide in dimethylsulfoxide, 1,8-diazabicyclo{5.4.0}undec-7-ene (hereinafter xe2x80x9cDBUxe2x80x9d), and lithium diisopropylamide (hereinafter xe2x80x9cLDAxe2x80x9d). The reformed mixture of the first and the second enantiomers of the 2xe2x80x2,3xe2x80x2-dideoxy-5-fluoro-3xe2x80x2thiocytadine derivative are separated by simulated moving bed chromatography such that the first enantiomer 2xe2x80x2,3xe2x80x2-dideoxy-5-fluoro-3xe2x80x2thiocytadine derivative is recovered in 95% enantiomeric excess, and the second enantiomer of the 2xe2x80x2,3xe2x80x2-dideoxy-5-fluoro-3xe2x80x2thiocytadine derivative or a mixture containing the second enantiomer of the 2xe2x80x2,3xe2x80x2-dideoxy-5-fluoro-3xe2x80x2thiocytadine derivative is recovered. In a preferred embodiment, the first enantiomer of 2xe2x80x2,3xe2x80x2-dideoxy-5-fluoro-3xe2x80x2thiocytadine derivative is recovered from the reformed mixture in at least about 90% yield.
In contrast to selective recrystallization, more than one stercoisomer can be collccted by the method of the invention in high diastereomeric or enantiomeric excess without additional processing steps. The methods of the invention also provide for reconverting, e.g., reracimizing, an undesired enantiomer into a racemic mixture which can be separated using simulated moving bed chromatography, and thus, greatly increasing the total recovery of a desired enantiomer.
The method of the invention results in an unexpectedly effective separation of stereoisomers of FTC derivatives, even those which exhibit relatively low solubilitics in many common solvents. The parameters determined for the process result in an excellent degree of separation for stereoisomers of FTC derivatives. Moreover, the separation can be achieved within the range of retention times available to most simulated moving bed systems.
Moreover, due to the fact that the methods of the invention require far less solvent than conventional separations, they are particularly suited for large scale operations. This process advantage further results in the products obtained from simulated moving bed separation being more concentrated and containing less solvent than those obtained using standard chromatographic techniques. Not only do such products require less post-separation treatment, such as evaporation of excess solvent, they are particularly suited for use in pharmaceutical preparations.